1. Field of the Invention
The present invention relates to a liquid ejection head having openings for ejecting liquid and an image-forming device using the same.
In this Specification, a word “print” refers to not only forming a significant information, such as characters and figures, but also forming images, designs or patterns on a printing medium and processing such as etching and so forth in the printing medium, whether the information is significant or insignificant or whether it is visible so as to be perceived by humans. The term “printing medium” includes not only paper used in common printing apparatus, but also sheet materials such as cloths, plastic films, metal sheets, glass plates, ceramic sheets, wood panels and leathers or three-dimensional materials such as spheres, round pipes and so forth which can receive the ink. The word “ink” should be interpreted in its wide sense as with the word “print”, refers to liquid that is applied to the printing medium for forming images, designs or patterns, processing such as etching in the printing medium or processing such as coagulating or insolubilizing a colorant in the ink and includes any liquids used for printing.
2. Description of the Related Art
Recently, demand for the high gradation color printing has risen as an internet or a digital camera becomes popular, and an ink jet printers having a higher performance have been developed therewith. The following methods (1) to (3) are known for obtaining a high precision, high gradation and high quality printed image:
(1) An arrangement pitch of openings for ejecting ink is minimized to facilitate the resolution.
(2) A plurality of print heads, each ejecting (at least two kinds of) a specific color ink containing a coloring material of different ratios; i.e., different color concentrations, are prepared and a deep ink and a light ink are selectively printed one over the other if necessary, so that the gradation is improved.
(3) By varying a size or an amount of an ink droplet ejected from the opening, the gradation is improved.
Since the above-mentioned method (3) is relatively difficult to be done in a so-called bubble-jet type printer in which a thermal energy is used for generating a bubble in the ink, a blowing pressure of which is used as an energy for ejecting ink from the opening of the print head, it is thought that the methods (1) and (2) are particularly effective for the bubble-jet type printer.
To realize the method (2), however, two or more print heads are necessary for a specific color ink to result in a high cost. Accordingly, for the bubble-jet type printer, it is most preferable and convenient to adopt a method in which the arrangement pitch of the ejection openings is reduced as in the method (1) and a size of an individual ink droplet ejected from the respective ejection opening is minimized (for example, to 10 picoliter or less) so that the resolution is improved. This is because the production cost hardly rises in this method.
A type for communicating a bubble to an atmosphere via the ejection opening when the small ink droplet is ejected from the ejection opening, which bubble is growing with the heating of ink due to the film boiling is disclosed, for example, in Japanese Patent Application Laid-open Nos. 4-10940 (1992), 4-10941 (1992) and 4-10942 (1992) To differentiate such a type from the conventional bubble-jet type in which the ink droplet is ejected without communicating the bubble growing due to the film boiling with the atmosphere, the former may be called as a bubble-through type.
In the print head of the conventional bubble-jet type in which the ink droplet is ejected without communicating the bubble growing due to the film boiling with the atmosphere, it is necessary to reduce a cross-sectional area of an ink passage communicating with the ejection opening as a size of the ink droplet ejected from the ejection opening becomes smaller. Thereby, an inconvenience may occur in that an ejection speed of the ink droplet is decelerated because of the lowering of ejection efficiency. If the ejection speed of the ink droplet decelerates, the ejecting direction becomes unstable. In addition, the ink is gradually viscous as a moisture is vaporized while the print head is inoperative to cause the ink-ejection to be further unstable, resulting in a premature ejection failure or others. As a result, the reliability may be lowered.
In this respect, the bubble-through type print head in which a bubble communicates with the atmosphere is suitable for ejecting an ink droplet, since a size of the ink droplet could be decided solely by a geometric configuration of the ejection opening. In addition, the bubble-through type print head is advantageous in that it is hardly affected by a temperature or others and an ejection rate of the ink droplet is very stable in comparison with the conventional bubble-jet type print head. Accordingly, it is possible to relatively easily obtain a high precision, high gradation and high quality printed image.
To obtain the high precision, high gradation and high quality printed image, preferably, an extremely small amount of ink droplet is ejected from an individual ejection opening during the printing operation. In this case, it is necessary to eject ink droplets from the ejection opening at a short period for the purpose of obtaining a high printing speed. Further, it is necessary to make a carriage carrying the print head thereon to scan at a high speed relative to a printing medium in synchronism with a drive frequency of the print head. On such a point of view, it could be said that the bubble-through type is particularly suitable for the ink jet printer.
A state of the ejection of ink droplet is depicted in FIG. 11, when a so-called “solid” printing is carried out on a printing medium, in which ink droplets are continuously ejected from all the ejection openings while subjecting the print head of such an ink jet type to the scanning movement at a high speed together with the carriage along the printing medium. The direction of the scanning movement of the print head 1 is vertical to a paper surface of FIG. 11, and the non-illustrated ejection openings are arranged leftward and rightward in the drawing. When the image data is “solid”, all of the ejection energy generating elements (not shown) corresponding to the respective ejection openings are driven at a high driving frequency. Therefore, viscous air around the ink droplet 3 ejected from the ejection opening toward the printing medium 2 is also entrained therewith. As a result, a surface area 4 of the print head 1 in which the ejection openings of the print head open is more decompressed than the periphery of the print head 1. Particularly, it has been found that the ink droplets 5 ejected from the ejection openings located at opposite ends of the opening arrangement are sucked toward a center along the arrangement, whereby the ink droplet is not directed to a predetermined position on the printing medium 2. It is apparent from the above-mentioned fact that a plurality of ink droplets ejected from the ejection openings disposed in the end section are drawn to a central section.
The effect of such a phenomenon is illustrated in FIG. 12. That figure schematically shows a solid printed image formed on the printing medium when the scanning movement of the carriage is repeated under the conditions described above with reference to FIG. 11. In FIG. 12, the carriage scans together with the print head from an upper area to a lower area in the drawing. As shown in FIG. 12, a white streak 7 is formed between a first part 5 of the solid printed image formed by a preceding scanning movement and a second part 6 of the solid printed image formed by a subsequent scanning movement.
Such an inconvenience is particularly significant in the bubble-through type ink jet printer having a small arrangement pitch of the ejection openings and capable of ejecting a small amount of ink droplet as little as 10 picoliter or less at a short period by one drive operation.
The relationship between an amount of ink droplets and an amount of end-deviation (half a width of the white streak 7) is illustrated in FIG. 13 when the arrangement pitch of the ejection openings is 21.2 μm (corresponding to 1200 dpi). A reason why such a phenomenon appears is that while a ratio of a surface area (a projected area) of the ink droplet relative to a weight of the ink droplet increases as a size of the ink droplet becomes smaller, the movement of the ink droplet is more largely influenced by an air stream.
To avoid this inconvenience, it is also possible to restrict the deflection of ejection trace of the ink droplet ejected from the ejection opening located at the respective opposite arrangement end by enlarging a size of the ink droplet; i.e., by increasing an inertia mass of the ink droplet, ejected from the ejection opening of the respective opposite arrangement end. The enlargement of the ink droplet size, however, causes the obstruction to the formation of a high precision and high gradation image. Further, the permeation of ink droplet into the printing medium is retarded, and the printed image is liable to deteriorate with the swell of the printing medium. Or, it is also possible to mitigate the above-mentioned inconvenience by suppressing the drive frequency for the ejection energy generating elements to a lower level. When the drive frequency for the ejection energy generating elements is set to a lower level, however, the printing speed becomes too slow to satisfy the user's need for obtaining a high speed printing.